as far as value and return per dollar, i take usage into the calculation when i figure if i'm spending foolishly.
i use the 0.10 per mile calculation... if i drive the car 100,000 miles and scrap it out at the end of it's tenure, i can invest 10,000 dollars into the vehicle over the 100,000 mile term.
for example, i own a 1995 dodge ram i bought new in 1994, paid $16k for it, and have driven it 208,000 miles at this point. I have probably spent another $2000 in tires/repairs over the life of the truck, so 18000 / 208000 = 8.6 cents/mile. I'm doing darn good w/ the truck.
Right now, i have approx $1800 invested in my capri and i've driven it around 1000 miles. If i sell it for $1700, I've hit my break even point. Or, in another light, if i want to sink another $4k into the car (which i probably will, i like the car) i need to get 60,000 miles of usage from it, or 30k and sell it for $3k etc....